In my experience, it is a common problem for taiji beginners to feel strain in the knee because the rules of Taijiquan say we should “rotate our hips”. When the kua (the technical term for the hip area) is not open enough, the knees tend to overcompensate and end up rotating. But the knees can only bend and unbend, they aren’t meant to rotate, so when they rotate, it leads to injury.
This can be prevented by paying attention to alignment. One simple way doing this is to keep the knee on top of the foot. Strain comes when the knee goes too far either on one side or on the other, or if it goes forward past the toes.
At a beginner level, the issue is potential injury. At a more advanced level, the issue becomes an inability to generate power
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thoroughly massaging around the kneecap after practice helps tremendously with this issue (until theyre very warm)…when your hands are swollen and red with chi…rub around the three acupunture points on either side and especially underneath….pressing the knees back as if hyperextending them (slightly!) opens circulation through the knee as well….. particularly for beginners with weak knees… i’ve seen people drop out of taiji because they dont cross the threshold of learning to relax into correct posture before they grind away at the knee joint itself… or too forcefully pounding mortar as an exuberant newbie…
“Both knee caps are flush with the heels” (The knee and front of the foot are not allowed to be level with each other) – Grand Master Hong Junsheng